Magnetic position and speed sensor having a hall probe

ABSTRACT

This invention relates to a position sensor featuring a stator defining an air gap inside of which a mobile magnet integral with a coupling shaft moves. The sensor features a Hall probe for measuring the variation of the induction in an air gap. The stator is formed of a first stationary part and a second part which is either stationary or mobile, the two parts defining between them a main air gap in which the part of the mobile element moves. The mobile element exhibits at least two adjacent thin parts magnetized crosswise in alternate directions, the magnetized parts being made of a material exhibiting in the entire working area a practically linear demagnetization characteristic and reversible permeability close to that of the air. The stationary part exhibits at least two secondary air gaps approximately perpendicular to the main air gap in which the mobile element moves. The Hall probe is housed in one of the secondary air gaps. The L/E ratio is greater than 6, L designation the width of a magnetic pole and E designating the width of main air gap.

This is a Continuation, of application Ser. No. 07/917,061 filed on Oct.1, 1992, now U.S. Pat. No. 5,528,139 which was filed as A 371 of PCTApplication No. PCT/FR91/00973 on Dec. 5, 1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a magnetic position sensor, and, according toa variant, a magnetic position and speed sensor.

2. Discussion of the Background

Position sensors using Hall probes detecting the magnetic flux generatedby a magnet by relative movement relative to the Hall probe are known inthe prior art. In particular, French patent 2624966 describes a coderfor a print wheel comprising a Hall-effect linear detector and apermanent magnet mounted in a nonferrous metal shaft exhibiting a ringforming a conductive flux spiral molded in the wheel and encircling theshaft. The angular position of the print wheel is determined in absolutevalue by the amplitude of the signal relative to the transition point ofthe spiral. In another embodiment, a second Hall-effect detector isplaced on the shaft, being opposite to the first detector to provide areinforced signal.

The sensors thus produced do not exhibit a signal which is actuallylinear, and, in the prior art, this drawback has been remedied bydigitizing the signal delivered by the Hall probe and by processing thesignal by data-processing means.

It has also been proposed in the prior art to remedy the defect oflinearity of magnetic position sensors by complex geometries. During theEuropean colloquium on the "modern magnets and new machines withmagnets" which was held in Grenoble on Jun. 13 to 15, 1990, a positionsensor delivering an output signal approximately proportional to theangular position was exhibited. This sensor comprised a magnetized ringof elliptical shape according to a first embodiment, or an originalqualified geometry consisting of two arcs of a circle. The linearityerror is thus cancelled by the eccentricity. If, on the theoreticalplane, this solution is advantageous, it is technically difficult toachieve for sensors produced industrially in large scale. The productioncost is thereby excessive for many applications.

SUMMARY OF THE INVENTION

The object of this invention is to remedy these drawbacks by proposingan angular or linear position sensor of low production cost, exhibitinga linearity error less than one percent and whose extent of measurementis slightly less than the length of a magnetic pole.

The position sensor according to this invention comprises a statordefining an air gap inside of which a mobile magnet integral with acoupling means moves. The sensor further comprises two secondary airgaps approximately perpendicular to the main air gap, a Hall probemeasuring the variation of the induction in at least one of thesecondary air gaps. The stator consists of a first stationary part and asecond part which can be either stationary or mobile. The two partsdefine between them said air gap in which the magnetized part of themobile element moves. The mobile element exhibits at least two thinparts magnetized crosswise in alternate direction made of a materialexhibiting in the entire working area a practically lineardemagnetization characteristic and a reversible permeability close tothat of the air. The stationary stator part exhibits two secondary airgaps perpendicular to the air gap in which the mobile element moves, aHall probe being housed in said secondary air gap. The L/E ratio isadvantageously greater than 6, and preferably greater than 10, where Ldesignates the linear width of the magnetic pole in the case of a linearsensor, or the length of the arc corresponding to the average radius ofthe pole, in the case of a rotary sensor and where E designates thewidth of the air gap. According to a particular embodiment, the mobileelement comprises only a single thin part magnetized crosswise,exhibiting the shape of a tile extending on 120° for sensor with arotary cylindrical rotor or mobile in translation, or the shape of ahalf-disk for a flat rotary sensor. The output signal is, however, ofweaker amplitude and poorer quality than in the embodiment using atleast two magnetized parts.

The Hall probe produces an electric signal proportional to the fluxdensity or induction which goes through it. So that this probe deliversa signal as a linear function of the angular or linear position, it isnecessary to place it in a magnetic field which varies as linearly aspossible with the position.

In the sense of this patent, stator designates the group of componentsof the circuits with high magnetic permeability, formed by a stationarypart, and a second part which in most cases is also stationary, but insome particular cases is integral with the mobile magnet.

According to a particular embodiment, the stator consists of a firststationary part and a second part integral with the magnetized part ofthe mobile element. Although this embodiment considerably increases thecharacteristic inertia of the sensor, it makes it possible to reinforcethe mechanical strength of the mobile element and therefore optionallyto use fragile magnetic materials.

According to a first variant, the stator consists of two coaxial rings,the outside ring comprising at least two radial air gaps in one of whichthe Hall probe is placed, the mobile element consisting of a magnetizedcylinder coaxial with said stator rings and mobile in rotation aroundthe axis of symmetry. The travel of the mobile element is C/2 on bothsides of a median position in which the transition zone between the twomagnetized parts of the mobile element is in the plane perpendicular tothe plane of symmetry of the radial secondary air gap in which the Hallprobe is housed, C being slightly less than π.

This embodiment makes it possible to produce small-sized sensorsdelivering an output signal proportional to the angular position with agreat precision and over a significant travel.

According to a second variant, the mobile element consists of a thindisk exhibiting two parts magnetized crosswise each extending overapproximately π, the stator consisting of a first stationary partexhibiting a secondary radial air gap in which a Hall probe is housedand a second stationary or mobile part consisting of a disk of a softmagnetic material.

Advantageously, the air gap is defined by a thrust ball bearing placedbetween the mobile element and the stationary stator part. Theattraction between the magnetized parts of the mobile element and thestationary stator part assures the positioning and the centering of themobile element. The movement produced by this attraction is limited bythe thrust ball bearing which makes it possible, however, to maintain acertain alignment flexibility and leads to a great resistance to shocksand vibrations.

According to a particular embodiment, the stationary stator partcomprises two Hall probes each housed in one of said secondary air gaps.

By summing the signals delivered by each of said Hall probes, thepossible geometry defects of the position sensor are attenuated.

According to a third variant, the stator consists of a stationary partexhibiting three stator poles and a second part defining between them anair gap in which the mobile element moves linearly.

According to a particular embodiment, the stationary stator part furthercomprises a sensing coil of the time variation of the magnetic fluxhoused in one of the secondary air gaps.

This coil delivers an electric signal proportional to the relative speedof the mobile element relative to said coil.

Advantageously, the stationary stator part further comprises a housingfor a temperature probe.

This probe makes it possible to compensate the variations of thecharacteristics of the magnetized parts and/or Hall probes under theeffect of the temperature and therefore to use components of moderatecost. This temperature probe can be housed in one of the secondary airgaps.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be understood better from reading the followingdescription, relating to several embodiments not constituting in any waya limitation of the scope of the invention and referencing the drawingswhere:

FIG. 1 represents a view in axial section of an angular position andspeed sensor,

FIG. 2 represents a view in median section of the same sensor,

FIG. 3 represents a view in median section of the same sensor in adifferent position,

FIG. 4 represents a view in axial section of a second embodiment of anangular sensor,

FIG. 5 represents a view in axial section of a third embodiment,

FIG. 6 represents a view in cross section of a linear sensor,

FIG. 7 represents a view in section of a linear sensor variant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sensor represented in FIG. (1) comprises a stator structurecomprising a first stationary part (1) and a second part (2) integralwith a tubular magnet (3). Two stator parts (1), (2) consist of a softmagnetic material with high permeability and low magnetic hysteresis. Byway of example, the stator parts can be made of annealed pure iron or ofan annealed iron-nickel alloy of the ANHYSTER D or MUMETAL type marketedby the Creusot Loire company, or else of sintered iron or iron-nickel.

Tubular magnet (3) is made by joining two parts magnetized radially inopposite direction or else by magnetization of a tubular part ofisotropic or anisotropic molded samarium cobalt 1/5 with radialorientation. Magnet (3) exhibits two pairs of poles, i.e. at itsperiphery, a north pole is seen on an angle slightly less than 180° anda south pole on the sector diametrically opposite, of an angle alsoslightly less than 180°. Two transition zones (6), (7), whose dimensionsdepend on the quality of the magnetization material and/or productiontechniques of this mobile element, are placed between the two pairs ofpoles.

The ring constituting second stator part (2) and tubular magnet (3) areassembled by gluing and are mounted on a coupling shaft (4).

Stationary stator part (1) is separated from mobile stator part (2) byan air gap (5) of a length E. As represented in FIG. 2, stationarystator part (1) consists of two half-rings (8), (9) made of a softmagnetic material preferably with high permeability and low hysteresis.Stator part (1) exhibits two radial cavities (10), (11) constitutingsecondary air gaps and in which a Hall probe (12) and an electric coil(13) are housed respectively. The length of these radial air gapsmeasured perpendicularly to the plane of symmetry passing through thecenter of the two air gaps is on the order of a millimeter.

Hall probe (12) is preferably of the type with an amplifier incorporatedin the same housing, which makes it possible to have only three wiresand to have an electric signal of greater amplitude, not very sensitiveto electric interferences. By way of example, the Hall probe is of theUGN 3503 type marketed by the Sprague company.

Electric coil (13) consists of a winding of axis perpendicular to thefaces of the stator defining secondary air gap (11). This windingcomprises several hundred turns of copper wire.

Main air gap (5) will be selected as small as possible, for example, fora sensor of average radius R of 5 millimeters at the center of the rotorand for a magnetized part of a thickness of 1 millimeter, the playbetween outside surface (15) of magnetized part (3) and inside surface(16) of stator part (1) will be on the order of 0.2 millimeter. In thisexample, the L/E ratio is 13,

L designating the width of a magnetic pole

L being slightly less than 5 mm×π=15.7 mm

E designating the dimension of air gap (5) or 1 mm+0.2 mm=1.2 mm.

In the position represented in FIG. 2, diametral plane (18) of themagnet passing through transition zones (6), (7) is angularly close toplane of diametral symmetry (17) of the stator passing through thecenter of secondary air gaps (10), (11). The difference is about 15°.Under these conditions, the major part of the flux produced by a pair ofpoles is enclosed through the secondary air gaps (10), (11) with thatproduced by the other pair of poles. If the mechanical embodiment isgood, each air gap conveys an equal flux, and the inductions in thesetwo secondary air gaps are equal and maximum. For an angle less than15°, transition zone (6), (7) approximates in an excessive way secondaryair gap, respectively (10), (11), and a magnetic discontinuitydisturbing the response of the sensor results from this.

In the position represented in FIG. 3, diametral plane (18) of themagnet passing through transition zones (6), (7) and plane of diametralsymmetry (17) of the stator passing through the center of secondary airgaps (10), (11) form an angle of 90°. In this case, no flux passesthrough secondary air gaps (10), (11) and the induction is zero there.For a larger angle on the order of 165°, it is clear that the inductionis equal in amplitude and sign opposite to the induction correspondingto FIG. 2. Thanks to the large L/E ratio and the characteristic of theselected magnet, the induction varies linearly as a function of theangular position on a range between 15° and 165°, or on about 150°.Stationary stator part (1) and mobile stator part (2) should have asufficient cross-sectional area not to saturate at any point. Moreover,to obtain a good linearity, the reluctance of the stator should be lowconsidering that of the secondary air gaps. It is clear that if theradius of magnet (3) is increased for the same dimension of the air gap,and for the same width of transition zone (6), (7), the travel withlinear response is increased and can reach values slightly less than180°, on the order of 170°. In the absence of a limit stop of the travelof the mobile element, the response of the sensor on a complete turnresembles a symmetrical sawtooth.

To reduce the driving torque of the sensor due to the variation of theoperating point of the magnet and to reduce the parasitic disturbancesproduced by the sensor on the measured system, it is preferable toreduce the variation of the operating point of the magnet andconsequently to reduce the reluctance of the air gaps in which the probeor probes are placed. For this purpose, it is advisable to choose a thinprobe and to increase the surface of this secondary air gap.

FIG. 4 represents a view in median section of another embodiment of asensor according to the invention. As in the example described above,the stator consists of a stationary part (1) whose shape isapproximately identical with the shape of the stationary part describedabove, and a disk-shaped mobile part (2). Magnet (3) consists of twohalf-washers magnetized crosswise in opposite direction. Magnet (3) isglued on one of the faces of mobile stator part (2). By way of example,the outside diameter of the magnet is 16 millimeters and the insidediameter is 8 millimeters, its thickness is one millimeter and the axialplay between the magnet and the closest surface of stationary statorpart (1) is about 0.3 millimeter. The average radius of magnet (3) is 6millimeters and the ratio between the width of the pole and thedimension of the air gap measured between the two stator parts is about15. Each magnetic pole has the shape of a circular sector of about 180°of opening. The magnetic forces attract the mobile element againststationary stator part (1). To prevent the contact between the mobileparts and the stationary part, the axial movement is limited by a thrustball bearing (20) whose ring (21) is magnetically isolated from stator(1) by a nonmagnetic part (22). The positioning and the guiding can beassured exclusively by the forces of magnetic attraction and thrust ballbearing (20), excluding any other mechanical guide. A great resistanceto shocks and vibrations, and a possibility of self-adaptation toalignment defects of coupling shaft (4) result from this.

FIG. 5 represents a view in section of a variant of the precedingembodiment. The position of the stator and the rotor have been reversedto simplify the assembly. Coupling shaft (4) is guided laterally, butnot axially, by a bearing (23).

In an extreme case, mobile part (2) exhibits a bore (40) deep enough tomake possible the housing of thrust ball bearing (41). In this case,stator (1) can consist of a solid disk exhibiting a single diametralslot.

In this case, the two air gaps are placed in the extension of oneanother and are reduced to a single slot. As a result, the concept of"two air gaps" will be extended in the sense of this patent to such aslot.

FIG. 6 represents a view in section of a linear actuator. Stationarypart (1) of the stator exhibits three magnetic poles, respectively (24,25, 26). Mobile stator part (27) is integral with a thin magnet (28)exhibiting two pairs of poles (29, 30) in opposite directions.Stationary stator part (1) exhibits a first secondary air gap (31) inwhich a Hall probe (33) is housed and a second secondary air gap (36) inwhich either an electric coil (34) or a second Hall probe is housed. Themedian plane of secondary air gaps (31, 36) is perpendicular to theplane of main air gap (5). The mobile element moves linearly, the usefultravel being limited by two end positions in which transition zone (35)between the two pairs of poles of magnet (3) reaches the vicinity of oneor the other of secondary air gaps (31), (36), "in the vicinity" meaningat a distance corresponding approximately to the width of main air gap(5).

In the example described, the thin magnet is plane. It can, of course,be made in various shapes, in particular in the shape of a tile orcylinder whose axis corresponds to the axis of the linear movement ofthe mobile element, stator parts (24, 25, 26) then being in the form ofcylindrical washers.

FIG. 7 represents a view in section of a linear sensor joined to anactuator (42) whose structure and operation are not explained.

Sensor stage (43) exhibits an axial symmetry relative to longitudinalaxis (44). It comprises a mobile element (45) integral with the axis ofthe actuator and comprising two annular magnets (46, 47) orientedradially, in opposite directions. The stator consists of threeferromagnetic parts (48, 49, 50) of annular shape. In this embodiment,the L/E ratio is slightly greater than 3. Two air gaps (51 and 52)extending radially make possible the positioning of Hall probes (53),(54).

This invention is not at all limited to the examples above, but extends,on the contrary, to all the variant embodiments.

We claim:
 1. A position sensor comprisinga stationary stator; a mobileelement located outside of the stationary stator, and comprising:acoupling shaft; and a magnet formed integral with the coupling shaft; afirst air gap being formed between the stationary stator and the mobileelement; two secondary air gaps formed in the stationary stator, each ofthe two stationary air gaps being approximately perpendicular to thefirst air gap; and a Hall probe housed in a first of the two secondaryair gaps; wherein a ratio L/E is greater than 3, where L designates awidth of a magnetic pole and E designates a width of the first air gap.2. The position sensor according to claim 1, wherein L/E is greater than6.
 3. A position sensor comprising:a stationary stator; a mobile elementlocated outside of the stationary stator, and comprising:a couplingshaft; and a magnet formed integral with the coupling shaft; a first airgap being formed between the stationary stator and the mobile element;two secondary air gaps formed in the stationary stator, each of the twostationary air gaps being approximately perpendicular to the first airgap; and a Hall probe housed in a first of the two secondary air gaps;wherein a sensing coil is housed in a second of the at least twosecondary air gaps.
 4. A position sensor comprising:a stationary stator;a mobile element located outside of the stationary stator, andcomprising:a coupling shaft; and a magnet formed integral with thecoupling shaft; a first air gap being formed between the stationarystator and the mobile element; two secondary air gaps formed in thestationary stator, each of the two stationary air gaps beingapproximately perpendicular to the first air gap; and a Hall probehoused in a first of the two secondary air gaps; wherein the magnetcomprises adjacent thin parts, magnetized crosswise in alternatedirection.
 5. A position sensor comprising:a stationary stator; a mobileelement located outside of the stationary stator, and comprising: acoupling shaft; and a magnet formed integral with the coupling shaft; afirst air gap being formed between the stationary stator and the mobileelement; two secondary air gaps formed in the stationary stator, each ofthe two stationary air gaps being approximately perpendicular to thefirst air gap; and a Hall probe housed in a first of the two secondaryair gaps; wherein the magnet comprises a thin disk exhibiting two partsmagnetized crosswise each extending over about π.
 6. A position sensorcomprising:a stationary stator; a mobile element located outside of thestationary stator, and comprising:a coupling shaft; and a magnet formedintegral with the coupling shaft; a first air gap being formed betweenthe stationary stator and the mobile element; two secondary air gapsformed in the stationary stator, each of the two stationary air gapsbeing approximately perpendicular to the first air gap; a Hall probehoused in a first of the two secondary air gaps; and a temperatureprobe, formed in a housing in the stationary stator.